Over the past several years, a class of cells commonly known as liquid cathode cells has emerged as the best candidate to provide substantially increased performance over the age-old zinc-carbon, alkaline and silver oxide cells. The liquid cathode cells are distinguished from more conventional cells in that the active cathode depolarizer is a liquid. The basic elements of the cell are an anode, typically consisting of an alkaline or alkaline earth metal, a current collector consisting of a high surface area material that is catalytically active in the reduction of the liquid cathode, a suitable separator located between the current collector and the anode and mechanically separating the two, and the electrolyte, which includes the liquid cathode as well as an ionically conductive solute dissolved in a liquid solvent. In certain type cells, the solvent performs the additional function of the active cathode depolarizer.
The liquid cathode cells that are most commonly discussed in the literature as having the best performance characteristics are those using lithium metal anodes and active cathode depolarizers that are either oxyhalides, such as thionyl chloride or sulfur dioxide. Sulfur dioxide has also been employed as the cell solvent, particularly in secondary or rechargeable cells of the type which are made the subject of the present invention.
Cells for contemplation herein employ a lithium anode and a solution of a highly soluble lithium salt as the electrolyte, such as lithium tetrachloroaluminate. Sulfur dioxide is used as the solvent, while the cathode is a high surface area carbon black containing cupric or copper II chloride.
Under ideal conditions, the cupric chloride contained within the porous carbon cathode discharges, displaying a single voltage plateau at approximately 3.4V. There is, however, a tendency for the cupric chloride to decompose to cuprous chloride via the reaction: EQU 2 CuCl.sub.2 .fwdarw.2 CuCl+Cl.sub.2
If impurity levels of copper I chloride are high, the cell displays additional voltage plateaus on charge and discharge. Typically, the above-referenced decomposition reaction results in between 2 and 10% cuprous chloride in the cathode, having the effect outlined above.
It is thus an object of the present invention to provide a rechargeable Li-SO.sub.2 cell having a cathode of porous carbon containing CuCl.sub.2 while minimizing the effects of CuCl contamination in the cathode.